1. Field of the Invention
This invention relates to the reflow or densification of glasses commonly employed as insulation and passivation layers in integrated circuits and particularly to methods of inducing flow or densification of these glasses.
2. Description of the Prior Art
Presently, phosphorus doped silicon oxide glasses (known as "phosphosilicate" glasses) are used in integrated circuits as insulation between the silicon material and integrated circuit leads and as insulation between various levels of conductive leads formed on integrated circuits containing multiple layers of conductive leads. In order to reduce the frequency of occurrence of cracks and breaks in the lead pattern resulting from steps in the height of the glass film, the glass film is heated to reflow and smooth out the steps. Typically, a phosphosilicate glass film comprises a chemical vapor deposited oxide of silicon doped with approximately 7% to 9% by weight of phosphorus. This film is formed in a well-known manner in a "CVD" reactor using chemical vapor deposition techniques involving an inert ambient or an ambient containing phosphorus dopant. In the latter case, the deposition of the glass film is carried out selectively in order to selectively dope with phosphorus some areas of the semiconductor material to comprise active regions of the device. The presence of phosphorous dopants in this situation is not directly related to the flow process to redistribute and smooth out the glass.
The main shortcoming of the prior art is that the structure must be heated to in excess of 1000.degree. C. in order to flow and densify the phosphosilicate glass. These elevated temperatures cause the dopants in the underlying semiconductor material to redistribute. Such redistribution is particularly undesirable and intolerable in the smaller and more advanced VLSI (very large scale intergration) devices now being developed where a small amount of redistribution causes a substantial change in device characteristics. The flow of phosphosilicate glass is discussed in a paper entitled "Phosphosilicate Glass Flow for Integrated Optics" by A. Naumaan and J. T. Boyd in the Journal of Vacuum Science Technology, January/February 1980.